Miniature horses: the veterinary issues you should know about

The main objective when breeding a miniature horse is to produce a tiny but proportionate horse. Registration as a British miniature horse requires a height of 87cm (8.2hh) or below at four years of age, so they’re certainly small — but are they always perfectly formed?

Owners should be aware that these diminutive equines can develop the same health conditions as larger breeds. Some conditions are more prevalent in miniatures, however, and often more problematic than in other horses.

With an average life expectancy of 30 to 40 years, a miniature can have a long and useful career in showing or driving, or as a companion animal. The key is to understand his unique physiology and adapt his care accordingly.

Potential problems

Miniatures have a degree of insulin insensitivity, which leads to poorer control of sugar and fat metabolism. This leaves them at greater risk of developing hyperlipaemia, a condition caused by a negative energy balance in the body. The resulting disturbance in the body’s fat metabolism leads to a build-up of triglycerides (fat products) in the blood.

In primary hyperlipaemia, this negative energy balance can be due to the increased demands of pregnancy and lactation, or by feed restriction — or even triggered by abnormal stress, such as losing a companion.

Secondary hyperlipaemia occurs as a consequence of anorexia, due to underlying disease that causes appetite loss.

Early detection of subtle clinical signs, which include lack of appetite, dullness and depression, can improve the prognosis. Even with intensive care, however, survival chances can be as low as 50%.

Prevention of hyperlipaemia is based on providing miniatures with appropriate nutrition all year round, paying special attention to pregnant and lactating mares or horses thought to require a restricted diet.

Dental care goes hand-in-hand with good diet. Overcrowding of cheek and incisor teeth is a major problem. This predominantly manifests itself between three and five years of age, due to the disproportionately small head and mouth compared to the size of the teeth at the time of eruption. Regular routine dental checks are, therefore, important from an early age.

Overcrowding of teeth can lead to tooth root problems, food impaction, gum disease and sometimes sinusitis. Early signs of potential problems in young animals are large, persistent and painful eruption bumps.

These hard swellings appear along the lower edge of the jaw, underneath the lower cheek teeth, and on the side of the face just above where the upper teeth would be. Offending teeth may need to be removed.

While miniatures are reported to be no more susceptible to colic than larger breeds, the type of colic they tend to develop is often different. Around 60% of cases are due to impaction and rock-hard faecal balls known as faecoliths.

This is thought to be related to the high incidence of dental problems and a relatively narrow portion of the intestines called the lumen.

Miniatures are inherently stoic, so early recognition of colic can be difficult. Rectal examination, an important diagnostic technique, is usually not possible due to size restraints, although ultrasound and radiographic examinations of the abdomen are relatively easier than in larger breeds.

Delayed diagnosis of faecoliths can be life-threatening, due to gut wall erosion and even rupture.

Exploratory laparotomy involving an incision in the abdomen wall may be required.

Current research seeks to determine how the diet and management of miniatures can be modified to minimise the occurrence of faecoliths and also enteroliths — accumulations of salts and minerals in the intestines.

Physical issues

Miniatures appear to be prone to primary osteoarthritis of the scapulohumeral (shoulder) joint, which may be associated with flattening of both the head of the humerus and the shallow depression it sits in.

Lameness typically results, with a characteristic low foot-flight, a shortened stride as the horse steps under his body, or both.

Manipulating the joint may cause obvious pain and muscle wastage may be evident. Treatment regimes currently available do not give a good outlook for a return to even paddock soundness and, in severe cases, euthanasia may need to be considered.

Another potential problem is hip dislocation, correctly termed coxofemoral luxation, which is rarely seen other than in miniatures. This can also occur in association with genetically predisposed lateral patella luxation (locking stifle). The small size of miniatures makes them good candidates for canine surgical techniques, with variable success and a guarded outlook for future soundness.

Miniature breeds also have increased incidence of tracheal collapse, which affects the windpipe and causes narrowing of the airway. This can happen in miniatures of all ages, often without any other respiratory disease, and is characterised by a respiratory noise that sounds like a goose honk.

Mild tracheal collapse may cause problems when the horse is exercised or stressed. In more severe cases, he may struggle to breathe at rest and his tongue and gums may turn blue due to lack of oxygen. In addition to rest and treatment of any lower airway disease, another option is to place a stent in the collapsed trachea — although this carries potential complications.

Breeding miniatures can be challenging. Early pregnancy diagnosis by ultrasound scanning via the rectum is often hard to achieve, which makes accurate prediction of foaling dates and early detection of twin pregnancies difficult. Fortunately, twinning rates in miniatures are

relatively low.

Because of an increased risk of dystocia (birthing difficulties), miniature broodmares should be monitored closely by CCTV at the time of foaling. An assisted birth may well be required.

Appropriate care, hoof trimming and regular exercise such as lungeing, long-reining or driving can head off other health issues. If he is stabled, a miniature will need a space measuring at least 8x8ft with a bottom door of a height that he can look over.

Take care that he is not bullied by bigger fieldmates by providing a low-railed area so he can have his own space. Ideally, graze him with other miniatures to allow him to interact with this own kind.

Defects and dwarfism

Very few figures are available on the prevalence and characterisation of congenital defects in miniature horses — a problem currently in the spotlight in the dog world.

The British Veterinary Association (BVA) works with the Kennel Club to identify and classify the severity of certain inherited conditions in dogs that are detrimental to their health. These include hip and elbow dysplasia and hereditary eye problems.

More recently, the BVA has launched a campaign associated with health and welfare concerns in brachycephalic dogs such as pugs, which feature a shortened skull shape.

These programmes are based on recognition of a real problem and an increase in its occurrence or severity, followed by the development of techniques for diagnostic testing.

Should the same happen for miniatures horses?

Flagging up and recording health problems could play a vital role in assisting research into the possible genetic links, ultimately leading to breeding programmes to address these issues.

Selection for small size can sometimes go wrong, resulting in the undesirable genetic trait of dwarfism and the associated limb deformities. Many miniature breed societies try to avoid accepting horses with known dwarfism breeding lines. Studies are currently being carried out with the aim of developing genetic testing for some types of dwarfism, to facilitate miniature horse breeding in the future.

Ref: Horse & Hound; 31 May 2018